On Mon, Jun 28, 2021 at 9:54 PM Tomas Pales <litewav...@gmail.com> wrote:
> On Monday, June 28, 2021 at 5:30:43 AM UTC+2 Bruce wrote: > >> >> Why should we ever be led to consider the set of all logically possible >> worlds? >> > > Because why does this particular world exist instead of some other? > It would seem that this is merely a philosophical issue, outside the domain of standard physics. For example, Lewis's analysis of counterfactuals in terms of possible worlds. As far as I can see, that is merely a linguistic trick. And what is the difference between a possible world that exists and a > possible world that doesn't, anyway? What does it mean "to exist"? I see no > difference between possibility and existence. > That is a serious conceptual shortcoming on your part. It is possible that there exists a horse-like creature with a single horn (unicorn), but that does not mean that unicorns exist anywhere outside the realms of the secret forest in Harry Potter novels. > >> I doubt that such a set can ever be well-defined. >> > > Maybe it can't. All possible concrete worlds might be identical to all > possible pure sets, which would need uncountably many axioms to define, as > per Godel's first incompleteness theorem. But there are some more limited > sets of possible worlds that are closely connected to known physics that > might be easier to define: possible worlds beyond the horizon of our > observable universe (but still in our universe), possible worlds of > inflationary multiverse, of string theory multiverse and of quantum > mechanical multiverse. > That is the set of worlds that have been suggested by some physical theories. The only one that really makes any sense is the existence of domains of space-time beyond our current Hubble horizon. There is reason to think that the initial singularity from which our universe evolved was of infinite (or at least unbounded) spatial extent. As the universe expands, galaxies and so on vanish over our Hubble horizon. We can never communicate with them again, but that does not mean that they cease to exist -- they are asimply in the greater regions of spacetime. But, at the same time, they necessarily have the same laws of physics as we do, so they do not constitute counterfactual "possible worlds" in that sense. There is no well-defined theory of eternal inflation, so other worlds from this source are purely speculative. String theory and its "landscape" is a failed attempt at a physical theory, and no credence can be placed in any supposed prediction of that theory. The quantum mechanical multiverse, arisong in the multi-worlds interpretation, must necessarily involve the same physics as we observe. And there is no reason to suppose that such worlds exist. MWI is not a well-worked out theory, despite several well-known attempts. Current evidence is against the existence of these other worlds -- we have >> evidence only for our world. >> > > What about worlds beyond the horizon of our observable universe (but still > in our universe)? By definition, we don't have direct observational > evidence for them. We do have indirect observational evidence that they > exist because they seem to be predicted by known physics. One might argue > that known physics also predicts some types of multiverse, although the > matters are not so clear there. > See the discussion of the various proposed multiverses above. Regions beyond our Hubble horizon which have the same physics as we observe constitute the only well-evidenced "other worlds" (if you want to call them that.) Bruce -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to everything-list+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/CAFxXSLTDeC5GcP0uP0prHmfjtFfag%2BkNOqF7t4a9YK03Bq-abA%40mail.gmail.com.
